1. Field of the Invention
The invention relates to a method and apparatus for determining the concentration of optically active substances, more particularly to an optical heterodyne-based method and apparatus which is suitable for noninvasive in vivo glucose monitoring of the aqueous humor in an eye of an animal.
2. Description of the Related Art
Optical noninvasive in vivo glucose monitoring using the aqueous humor glucose in an eye of an animal as a measure of the blood glucose concentration is known in the art. Rabinovitch, B., March, W. F., and Adams, R. L., have described an optical glucose monitoring scheme in "Noninvasive Glucose Monitoring of the Aqueous Humor of the Eye: Part I. Measurement of Very Small Optical Rotations", Diabetes Care, Vol. 5, No. 3; pp. 254-258, May-June 1982, and in "Noninvasive Glucose Monitoring of the Aqueous Humor of the Eye: Part II. Animal Studies and the Scleral Lens", Diabetes Care, Vol. 5, No. 3; pp. 259-265, May-June 1982. In the proposed scheme, a polarized incident light beam is modulated by a Faraday day effect modulator before being directed laterally through the anterior chamber of the eye. The light beam through the anterior chamber then passes through a Faraday effect path-length compensator, a crossed analyzing polarizer; and to a light detector. The output of the detector is received by a frequency-selective amplifier, which generates an amplified voltage that is a direct measure of the optical activity of the glucose in the aqueous humor present in the anterior chamber for use in determining the glucose concentration. A feedback mechanism may be included for increased sensitivity.
It is noted that the aforementioned glucose monitoring scheme has a complex construction due to its use of a Faraday affect modulator and path-length compensator, and a feedback mechanism for reducing system instability of the modulator. In addition, noise susceptibility limits the accuracy of the aforementioned scheme.